The present invention relates to techniques of an X-ray CT apparatus which optimizes spatial resolution and image noise of two-dimensional distribution tomographic images indicative of X-ray tube voltage-dependent information at X-ray absorption coefficients related to a distribution of atoms, tomographic images subjected to a so-called dual energy scan in a medical X-ray CT (Computed Tomography) apparatus or the like, and of an X-ray CT image reconstructing method used therefor.
In an X-ray CT apparatus using a two-dimensional X-ray area detector, tomogram imaging based on a low X-ray tube voltage and a high X-ray tube voltage has heretofore been performed upon obtaining tomographic images of a two-dimensional distribution of given atoms as shown in FIG. 4 to thereby obtain a tomographic image at the low X-ray tube voltage and a tomographic image at the high X-ray tube voltage. Then, the tomographic image at the low X-ray tube voltage and the tomographic image at the high X-ray tube voltage are subjected to a weight adding process in accordance with weighted addition coefficients prescribed or defined every atom for the two-dimensional distribution desired to see, whereby a two-dimensional distribution indicative of X-ray tube voltage-dependent information at X-ray absorption coefficients related to the distribution of the given atoms, tomographic images subjected to a so-called dual energy scan were obtained. As shown in FIG. 5, a low X-ray tube voltage and a high X-ray tube voltage are applied to obtain X-ray projection data of the low X-ray tube voltage and X-ray projection data of the high X-ray tube voltage. Then, the X-ray projection data of the low X-ray tube voltage and the X-ray projection data of the high X-ray tube voltage are subjected to a weight adding process in accordance with the weighted addition coefficients defined every atom to obtain tomographic images subjected to the so-called dual energy scan or imaging.
However, even in the case of the weight adding process for the tomographic images in FIG. 4 and the weight adding process for the X-ray projection data in FIG. 5, one of the weighted addition coefficients is a minus number. Therefore, image noise of the tomographic images subjected to the so-called dual energy scan, corresponding to the tomographic images indicative of the X-ray tube voltage-dependent information tend to become worse than image noise of the tomographic image at the original low X-ray tube voltage and image noise of the tomographic image at the high X-ray tube voltage. Therefore, it was difficult to ensure image quality of the final tomographic image subjected to the dual energy scan, i.e., ensure spatial resolution and image noise. Therefore, when the application of X rays is increased to improve the image quality, X-ray exposure increases. A problem arises in that when the dose of X rays is decreased in revere, the image quality falls short.
Patent Document 1. Japanese Unexamined Patent Publication No. 2003-244542.